This invention relates to a multilayer roll having a thin fluoropolymer top coat on an elastomeric base layer bonded to a rigid insert and methods of preparation of the roll which is particularly well suited for use in xerographic and electro photographic copying machines and printers.
There are a wide variety of rolls used in xerographic copiers and electro graphic printing devices. Typically, these are fabricated with a solid core or insert and have an intermediate elastomeric base layer covered by a smooth top coat, such as a fluoropolymer top coat. The top coat provides an outer surface area of low surface tension. In operations such as fusing, which are generally carried out at elevated temperatures, the low surface tension prevents toner from adhering to the roll surface, or reduced image quality and offset when toner attaches to the roll surface.
Among the various types of rolls utilized in these devices, include fuser, pressure and donor rolls, film forming and drying rolls in wet or liquid toner systems, corona rolls, squeegee rolls, photoconductor rolls, low friction rolls and printing rolls. Thus, there is a large need for rolls where the properties of the top coat can be varied to serve these various end uses. This includes rolls having improved release properties, extended useful life and protect the intermediate elastomeric base layer which is bonded to the roll insert. These needs arise particularly in the case of higher speed applications.
In order to retain the advantages of the elastomeric base coat such as good compliance and efficient heat transfer, it is desirable to provide as thin a fluoropolymer top coat as possible. There are several ways in which a fluoropolymer covered roll with an elastic base layer can be fabricated. Several of these are well known and used for producing rolls used in the electro photographic industry.
The most common way to prepare a roll having a fluoropolymer top coat on an elastomeric base layer is to heat shrink a pre-expanded fluoropolymer tube over a cured elastomer. A more elaborate method uses a xe2x80x9cmold in placexe2x80x9d technique. Here, an insert is centered in a fluoropolymer tube and elastomeric material is injected between the insert and the outer fluoropolymer tube. In this method, the fluoropolymer tube is generally flexible, yet self-supporting and is typically between about 254 to 635 microns (15 and 25 mils) in thickness. In U.S. Pat. No. 3,613,168 to Rowland and Tabelle a flexible yet self- supporting relatively thick fluoropolymer tube is placed inside a groove in an end piece of a casing which is mounted on a spindle of an extruder to maintain the core and fluoropolymer sleeve concentrically. Silicone rubber is then extruded into the space between the mounted tube and core, which has been mounted on the spindle so that the elastomer material bonds to the core and fluoropolymer sleeve after curing.
Other methods of manufacture include stretching a fluoropolymer sleeve below its yield point and while at this stage, inserting an elastomeric covered roll into the stretched sleeve. Releasing the pressure on the sleeve allows the fluoropolymer sleeve to shrink back and come into complete contact with the roll. In this case, the diameter of the elastomeric roll is larger than the unstretched fluoropolymer sleeve. When a vacuum is used in this process, it provides the added advantage of an air free fluoropolymer/elastomer interface. Alternatively, a fluoropolymer powder or latex can be sprayed or coated onto an elastomeric base layer.
In European Patent Application EP 625 735 to Japan Gore-Tex rolls are provided with a release surface of porous polytetrafluoralethylene top coat in which the pores are impregnated with a cross-linked synthetic rubber over an elastic porous body material of synthetic rubber foam. Here, the thickness of the PTFE release surface can be as low as three microns. The roll is fabricated by applying a non-continuous adhesive layer on the outer surface of the porous silicone rubber base coat and wrapping a porous expanded PTFE film of about 20 microns thick with a pore size of about two microns, one turn about the silicone rubber foam to form a single layer with a slight edge overlap. The entire assembly is then heated to fuse the adhesives and adhere the pores expanded PTFE film to the silicone rubber.
Thin films of porous PTFE impregnated with uncured silicone rubber for forming release top coats having thicknesses as low as three microns are also described in European Patent No. EP 441 114 to Fuji Xerox Co., Ltd. and Japan Gore-Tex, Inc. Here, unbaked polytetrafluoralethylene is expanded forming a fibrillated polytetrafluoralethylene film with voids ranging from 30 to 98 percent and pores from 0.02 microns to 15 microns is soaked with one or more types of silicone rubber to fill the voids. The film is wrapped around the surface of the roll having an elastic body layer and thereafter hardened by heating.
While these rolls and methods of fabrication described in the prior art are generally suitable for producing rolls with PTFE top coats, it remains desirable to provide an improved roll having an expanded PTFE top coat and an improved method of fabricating such rolls.
Generally speaking, in accordance with the invention, a thin porous fluoropolymer membrane in sheet or sleeve form for forming a roll with a thin fluoropolymer top layer over an elastomeric base layer bonded to a substantially rigid insert is provided. The fluoropolymer member is formed from an extruded film or a sheet which is stretched in both machine and traverse directions, taken to its melting point and at least partially sintered. When the fluoropolymer membrane is in the form of a film, it is wrapped in layers directly over an insert which has been positioned in end spiders with the film wrapped about the insert and extending past the end of the insert to cover the spiders. The top coat and insert assembly is placed into a concentric thin walled molding sleeve which is then placed into the cavity of the book mold. Liquid or flowable solid uncured elastomer material is injected through a spider mounted at the end of the insert into the space between the insert and top coat membrane causing the membrane to expand and fill a tubular mold cavity. Upon application of heat and pressure, the elastomeric material is cured. The wrapped fluoropolymer layers are compressed creating a continuous covering with no evident seam line. Depending on the intended application of the roll, the top fluoropolymer layer can be sintered to a desired degree or remain non-sintered.
In an alternative embodiment, a thin flexible non-self supporting extruded tube of fluoropolymer is utilized in place of the wrapped membrane. Here, the thin non-self supporting tube is fed over the insert and end spiders extending beyond the length of the insert and spiders. The assembled insert and top coat is placed into a tube of a book mold and the uncured elastomeric material is injected between the insert and thin fluoropolymer sleeve expanding and sizing the sleeve to the internal diameter of the tube. The elastomeric material is cured, bonding to the insert and the top coat to the cured elastomer.
The method of fabrication of the roll includes the step of placing the membrane or sleeve over the insert and extending the top coat over the end spiders. This allows injection of resin between the insert and top coat without teasing or wrinkling when injecting the elastomer as the top coat is a non-self supporting sheet or tube. This permits fabrication of rolls having fluoropolymer top coat as thin as three microns.
Accordingly, it is an object of the invention to provide a method of fabricating rolls with a fluoropolymer top layer over an elastomeric base layer bonded to an insert.
Another object of the invention is to provide an improved apparatus for fabricating a roll with a thin fluoropolymer top layer over an elastomeric base layer bonded to an insert.
A further object of the invention is to provide a roll having a thin fluoropolymer top layer from a PTFE expanded sheet or tube having a thickness from about 5 to 50 microns (0.2 to 2.0 mil) on an elastomeric base layer bonded to an insert.
Yet a further object of the invention is to provide an improved method for forming a roll having a thin fluoropolymer top layer over an elastomeric base layer bonded to an insert from an extruded film of the fluoroelastomer.
Yet, another object of the invention is to provide an improved roll with a thin fluoropolymer top layer over an elastomeric base layer bonded to an insert using a thin, non-self supporting extruded sheet or tube of fluoropolymer.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, the apparatus embodying features of construction, combination(s) and arrangement of parts which are adapted to effect such steps, and the product which possesses the characteristics, properties, and relation of constituents (components), all as exemplified in the detailed disclosure hereinafter set forth, and the scope of the invention will be indicated in the claims.